- What is thiomersal?
Thiomersal (also known as thimerosal or
mercurothiolate) is a
mercury-containing compound used in trace amounts to prevent bacteria and other organisms
from contaminating vaccines, especially in opened multi-dose vials.
- Is thiomersal a new component of vaccines?
No, it is not new. Thiomersal has been
used for about 60 years in the manufacture of certain vaccines. There have been no reports
of adverse events due to thiomersal except for very rare reports of sensitization (skin
- What is WHO doing about thiomersal in vaccines?
- WHO is working to bring about a
move away from the use of thiomersal in vaccines when an effective alternative becomes
available, and is working with national regulatory authorities and manufacturers to bring
- WHO is explaining that the risk to unvaccinated children of death and complications
from vaccine-preventable diseases in is real and enormous. The risk from side effects of
thiomersal is theoretical, uncertain and, at most, extremely small.
- WHO is aiming to replace thiomersal with other preservatives in the long term.
- WHO is examining possibilities for combining several vaccines together in one vial.
In this way, the same quantity of thiomersal can act as a preservative for several
vaccines at once, thereby reducing to an absolute minimum the amount of mercury given to
- When was it determined that high levels of thiomersal content in vaccines is a potential
The level of mercury contained in three doses of DTP, hepatitis B and Hib
vaccines (9 doses in all) does not exceed the WHO limit for mercury intake over the first
12 months of life. It was noticed by officials in the United States recently that infant
immunizations could theoretically raise mercury levels above those set by certain US
- Do all vaccines contain thiomersal?
No, not all vaccines contain
of the "live" vaccines contain it: measles, MMR, oral polio, yellow fever,
Many of the other vaccines in single-dose vials do not contain it either. Injectable polio
vaccine is also free from thiomersal. Triple vaccine (DTP) may not contain thiomersal if
the pertussis component is the acellular vaccine.
- Is there a replacement for thiomersal?
There are a number of other chemicals such
as 2-phenoxyethanol used as vaccine preservatives. But none does the job as well as
thiomersal. Even if others are found appropriate, it will take time to introduce them.
Replacing the preservative in a vaccine means it becomes a "new product" for
licensing purposes and has to go through the pathway to be tested and fully licensed. It
is exactly this pathway which ensures vaccines are as safe as possible.
- Do vaccine package inserts contain information about whether thiomersal is in the
They may not contain this information. Inserts of vaccines procured by WHO
and UNICEF already contain this information.
- Who is most at risk from mercury?
Mercury is not good for anyone. But those most
at risk are unborn babies and newborn babies. The brains of these little ones are the
organs most at risk from mercury. The type of mercury in thiomersal is not the same type
or form as that causing poisonings during industrial accidents. It is not as toxic because
it is bound to other chemicals in the thiomersal molecule and is not free to react with
the human body in the same way. The amount of mercury in vaccines is minute compared with
the amount generally involved in accidents.
- What should parents do (vaccinate or not vaccinate with thiomersal-containing vaccines)?
For children in industrialized and developing countries alike, the risk of death
and complications from vaccine-preventable diseases is real and enormous. The risk from
side effects of thiomersal is theoretical, uncertain and, at most, extremely small. The
best advice to parents is to continue having their children vaccinated. The balance of
risk is clearly in favour of continuing to vaccinate. Ensuring the quality and safety of
vaccines is a continuous process for WHO, national regulatory authorities and
- What proof do parents have that vaccinating their children is safe? What scientific
information do you have to backup your recommendation to vaccinate?
containing thiomersal have been used for 60 years without bad effects being noted from
thiomersal. There is no information showing that thiomersal in vaccines is dangerous. The
discussions currently are about theoretical risks.
- What should doctors and vaccinators tell worried parents?
Parents should be told
as much as they want to know and are able to understand. The issues are very complex and
most parents do not want to know all the science they generally want to be
reassured something is safe or not safe by a person they trust.
- Can all vaccines be made thiomersal-free? If so, how quickly?
Most vaccines could
be made thiomersal-free quite quickly, but they would not contain a preservative. It is
not safe to use multi-dose vials of certain vaccines without some form of preservative.
One solution would be to use single-dose vials, but this solution is very expensive and
not always technically possible. If a new preservative were to be used, the product would
have to be relicensed, taking a long time. Equally if thiomersal was removed from a
vaccine, it would have to be re-licensed as well.
- Are children who have been given vaccines with thiomersal at risk for adverse side
effects? What are the possible side-effects? What kind of treatment can a child receive?
is a potential risk of a skin sensitization producing a rash with any mercury-containing
products. The amount of thiomersal in vaccines is very small, and the risk of adverse
reactions are remote and theoretical. However, high levels of mercury in pregnant mothers
may result in brain damage to the unborn baby. This has happened rarely in the past when,
for instance, grain contaminated by mercury was eaten. Children who have received
thiomersal products do not need treatment.
- How many children in the world have been given more than the recommended amount of
thiomersal through vaccines?
WHO estimates that no child receives more than the WHO
recommended dose of mercury from vaccines alone in the first year of life. Some
communities where a lot of fish is eaten (fish may contain a high mercury content) may
raise their mercury levels above the recommended levels.
- In light of this information, would you vaccinate your own children right now with
Yes. I was vaccinated with triple vaccine as a
child and my own children have been given triple vaccine and hepatitis B vaccine. Knowing
about thiomersal would not change those decisions for me.
- Should other countries follow the advice given to the American public regarding
hepatitis B vaccination of the newborn?
The advice to the American public makes
good sense for the United States. But for developing countries where the risk of hepatitis
B being transmitted from mother to baby is very high, WHO current recommendations should
continue. WHO does not recommend screening before vaccination in national vaccination
schedules. There are millions of new cases of hepatitis B each year, resulting in
thousands of deaths in adults annually. The risk from the disease is huge, the risk from
the presence of thiomersal in the vaccine is theoretical, uncertain, and at most,
- Will there be a global shortage of vaccines now?
WHO, UNICEF and other public
health authorities all consider there is no reason to stop using current vaccines. If
public opinion accepts this, there will not be a problem. If opinion-leaders create a
panic situation, the public might refuse to use thiomersal-containing vaccines.
Manufacturers would then not be able to turn to alternative products to meet the needs of
the world's children.
- Will children in the developing world suffer?
WHO and UNICEF are fully committed to avoiding any hardship, suffering or disadvantage for
children in the developing world as a result of the thiomersal discussion. The children
have enough to put up with already.